This invention relates in general to direct write, processless imaging members, and particularly to heat-sensitive imaging members, that require no wet processing after imaging. The invention also relates to methods of digital imaging and printing using these imaging members.
The art of lithographic printing is based upon the immiscibility of oil and water, wherein an oily material or ink is preferentially retained in certain areas and the water or fountain solution is preferentially retained in other areas, depending upon the type of image produced. When a suitably prepared surface is moistened with water, and ink is then applied, either a positive or negative image is obtained, depending upon whether the imaged or non-imaged areas are ink-accepting. The ink is eventually transferred to the surface of a suitable substrate, such as cloth, paper or metal, thereby reproducing the image.
Very common lithographic printing plates include a metal or polymer support having thereon an imaging layer sensitive to visible or UV light. Both positive- and negative-working printing plates can be prepared in this fashion. Upon exposure, and perhaps post-exposure heating, either imaged or non-imaged areas are removed using wet processing chemistries.
Thermally sensitive printing plates are less common. Examples of such plates are described in U.S. Pat. No. 5,372,915 (Haley et al). They include an imaging layer comprising a mixture of dissolvable polymers and an infrared radiation absorbing compound. While these plates can be imaged using lasers and digital information, they require wet processing using alkaline developer solutions.
Conventional preparation and use of such printing plates generally involves multiple processing steps such as exposure to either light or heat (or both) through a patterned image, and wet processing with an alkaline developer to provide a printing plate image.
Dry planography, or waterless printing, is well known in the art of lithographic offset printing and provides several advantages over conventional offset printing. Dry planography is particularly advantageous for short run and on-press applications. It simplifies press design by eliminating the fountain solution and aqueous delivery train. Careful ink water balance is unnecessary, thus reducing rollup time and material waste. Silicone rubbers [such as poly(dimethylsiloxane) and other derivatives of poly(siloxanes)] have long been recognized as preferred waterless-ink repelling materials. The criteria for waterless lithography and the ink repelling properties of poly(siloxanes) have been extensively reviewed in the TAGA Proceedings 1975 pages 120, 177 and 195 and 1976 page 174. It was concluded that, in addition to low surface energy, the ability to swell in long-chain alkane ink solvents (i.e., its xe2x80x9coleophilicxe2x80x9d nature) accounts for silicone""s superior ink releasing characteristics. An important consideration is that siloxane polymers repel ink.
It has been recognized that a lithographic printing plate could be created containing an IR absorbing layer. Canadian 1,050,805 (Eames) discloses a dry planographic printing plate comprising an ink receptive substrate, an overlying silicone rubber layer, and an interposed layer comprised of laser energy absorbing particles (such as carbon particles) in a self-oxidizing binder (such as nitrocellulose) and an optional cross-linkable resin. Such plates were exposed to focused near IR radiation with a Nd++YAG laser. The absorbing layer converted the infrared energy to heat thus partially loosening, vaporizing or ablating the absorber layer and the overlying silicone rubber. The plate was developed by applying naphtha solvent to remove debris from the exposed image areas. Similar plates are described in Research Disclosure 19201, 1980 as having vacuum-evaporated metal layers to absorb laser radiation in order to facilitate the removal of a silicone rubber overcoated layer. These plates were developed by wetting with hexane and rubbing. CO2 lasers are described for ablation of silicone layers by Nechiporenko and Markova, PrePrint 15th International IARIGAI Conference, June 1979, Lillehammer, Norway, Pira Abstract 02-79-02834. Typically, such printing plates require at least two layers on a support, one or more being formed of ablatable materials.
xe2x80x9cDirect writexe2x80x9d imaging eliminates the use of the pattern of light or heat to generate an image. When a laser is used for this purpose, the laser can be used to heat only small regions at a time. Moreover, a computer can be used to produce the high resolution images pixel by pixel. If the plate is processless, chemical development is also eliminated.
While the noted printing plates used for digital, processless printing have a number of advantages over the more conventional photosensitive printing plates, there are a number of disadvantages with their use. The process of ablation creates debris and vaporized materials that must be collected. The laser power required for ablation can be considerably high, and the components of such printing plates may be expensive, difficult to coat, or unacceptable in resulting printing quality. Typically, such printing plates require at least two layers on a support, one or more being formed of ablatable materials.
Thermally switchable polymers have been described for use as imaging materials in printing plates. By xe2x80x9cswitchablexe2x80x9d is meant that the polymer is rendered either more hydrophilic (or oleophobic) or hydrophobic (or oleophilic) upon exposure to heat.
As an alternative method of preparing printing plates, U.S. Pat. No. 4,634,659 (Esumi et al) describes imagewise irradiating hydrophobic polymer coatings to render exposed regions more hydrophilic in nature. While this concept was one of the early applications of converting surface characteristics in printing plates, it has the disadvantages of requiring long UV light exposure times (up to 60 minutes).
EP-A 0 652 483 (Ellis et al) describes lithographic printing plates imageable using IR lasers, and which do not require wet processing. These plates comprise an imaging layer that becomes more hydrophilic upon the imagewise exposure to heat. This coating contains a polymer having pendant groups (such as t-alkyl carboxylates) that are capable of reacting under heat or acid to form more polar, hydrophilic groups. The problem with such materials is that they are very difficult to manufacture, exhibit poor shelf life, require a photoacid generator for imaging, and are positive-working only. Other lithographic printing plates hydrophilic polymers containing pendant carboxylic acids are described in U.S. Pat. No. 4,081,572 (Pacansky).
Positive-working photoresists and printing plates having crosslinked, UV-sensitive polymers are described in EP-A 0 293 058 (Shirai et al). The polymers contain pendant iminosulfonate groups that are decomposed upon UV exposure, generating a sulfonic group and providing polymer solubility.
U.S. Pat. No. 5,512,418 (Ma) describes the use of cationic polymers containing pendant ammonium groups for thermally induced imaging. However, chemical processing is still required to provide the desired image.
Japanese Kokai 9-197,671 (Aoshima) describes a negative-working printing plate and imaging method in which the imaging layer includes a sulfonate-containing polymer, an IR radiation absorber, a novolak resin and a resole resin. Wet processing with a conventional alkaline developer is required to produce the desired negative image.
Thus, the graphic arts industry is seeking alternative means for providing a processless, direct-write, positive-working lithographic printing plate that can be imaged without ablation and the accompanying problems noted above.
The problems noted above are overcome with a positive-working imaging member comprising a support having thereon a heat-sensitive surface imageable layer comprising:
a) a heat-sensitive polymer comprising a heat-activatable sulfonate group, and
b) a photothermal conversion material, the heat-activatable sulfonate group represented by structure I: 
wherein X is a divalent linking group, Xxe2x80x2 is an oxygen or a sulfur atom, and Y is an imino, an alkyl group, or an amide group.
This invention also includes a method of imaging comprising the steps of:
A) providing the positive-working imaging member described above, and
B) imagewise exposing the imaging member to thermal energy to provide exposed and unexposed areas on the surface of the imaging member, whereby the exposed areas are rendered more hydrophilic than the unexposed areas.
Preferably, the method is carried further with the step of:
C) without wet processing after imaging, contacting the imagewise exposed imaging member with a lithographic printing ink, and imagewise transferring the ink to a receiving material.
The positive-working imaging member of this invention has a number of advantages, thereby avoiding the problems of previous printing plates. Specifically, the problems and concerns associated with ablation imaging (that is, imagewise removal of surface layer) are avoided because imaging is accomplished by xe2x80x9cswitchingxe2x80x9d the exposed areas of its printing surface to more hydrophilicity, or oil-repellency. The resulting imaging members display high ink receptivity in non-exposed areas, good chemical resistance and excellent ink/water discrimination. No wet chemical processing (such as processing using an alkaline developer) is needed with the practice of this invention to remove portions of the surface imageable layer.
These advantages are achieved by using a specific heat-sensitive polymer in the surface imageable layer. These polymers have heat-activatable sulfonate groups either in the polymer backbone or pendant thereto. Such heat-activatable groups can include sulfoimino groups, or sulfoalkyl groups substituted with electron withdrawing groups that are described in more detail below.
In the lithographic art, materials that release or repel oil-based inks are usually referred to as having xe2x80x9coleophobicxe2x80x9d, xe2x80x9chydrophilicxe2x80x9d or ink-repelling character and, conversely, the terms xe2x80x9coleophilicxe2x80x9d and xe2x80x9chydrophobicxe2x80x9d are used to describe ink loving or accepting materials.
The imaging members of this invention comprise a support and a surface imageable layer thereon that contains a heat-sensitive composition. The support can be any self-supporting material including polymeric films, glass, metals or stiff papers, or a lamination of any of these three materials. The thickness of the support can be varied. In most applications, the thickness should be sufficient to sustain the wear from printing and thin enough to wrap around a printing form. A preferred embodiment uses a polyester support prepared from, for example, polyethylene terephthalate or polyethylene naphthalate, and having a thickness of from about 100 to about 310 xcexcm. Another preferred embodiment uses a metal (such as aluminum) sheet having a thickness of from about 100 to about 600 xcexcm. The support should resist dimensional change under conditions of use. The aluminum and polyester supports are most preferred for lithographic printing plates.
The support may be coated with one or more xe2x80x9csubbingxe2x80x9d layers to improve adhesion of the final assemblage. Examples of subbing layer materials include, but are not limited to, adhesion promoting materials such as alkoxysilanes, aminopropyltriethoxysilane, glycidoxypropyltriethoxysilane, epoxy functional polymers and ceramics, as well as conventional subbing layer materials used on polyester supports in photographic films. One or more IR radiation reflecting layers, such as layers of evaporated metals can be incorporated between the heat-sensitive layer and the support. In addition, an anti-IR radiation reflection layer can be incorporated in the imaging member if desired.
The back side of the support may be coated with antistatic agents and/or slipping layers or matte layers to improve handling and xe2x80x9cfeelxe2x80x9d of the imaging member.
The imaging member, however, has a surface layer that is required for imaging. This surface imageable layer consists essentially of one or more heat-sensitive polymers as described below, and a photothermal conversion material (described below), and provides the outer printing surface. Because of the particular heat-sensitive polymer(s) used in the imageable layer, the thermally exposed (imaged) areas of the layer are rendered more hydrophilic in nature. The background (unexposed) areas then remain more oleophilic.
In the heat-sensitive layer of the imaging members of this invention, only the heat-sensitive polymer and photothermal conversion material are necessary or essential for imaging. Thus, they are the only essential components of the imageable layer.
Each of the heat-sensitive polymers useful in this invention has a molecular weight of at least 5000, and preferably of at least 8000. The polymers can be vinyl homopolymers or copolymers prepared from one or more ethylenically unsaturated polymerizable monomers that are reacted together using known polymerization techniques, or they can be condensation type polymers (such as polyesters, polyimides, polyamides or polyurethanes) prepared using known polymerization techniques. Whatever the type of polymers, at least 25 mol % of the total recurring units comprise the necessary heat-activatable sulfonate groups.
The heat-sensitive polymers useful in the practice of this invention can be represented by the structure II wherein the sulfonate group is a pendant group: 
wherein A represents a polymeric backbone, X is a divalent linking group, Xxe2x80x2 is an oxygen or a sulfur atom, and Y is an imino, an alkyl group, or an amide group. Useful xe2x80x9cXxe2x80x9d linking groups include substituted or unsubstituted alkylene groups having 1 to 6 carbon atoms (such as methylene, ethylene, n-propylene, isopropylene and butylenes) that can have one or more oxygen, nitrogen or sulfur atoms in the chain, substituted or unsubstituted arylene groups having 6 to 10 carbon atoms in the aromatic ring (such as phenylene, naphthalene and xylylene), substituted or unsubstituted arylenealkylene (or alkylenearylene) having 7 to 20 carbon atoms (such as p-methylenephenylene, phenylenemethylenephenylene, biphenylene and phenyleneisopropylene-phenylene), or the group xe2x80x94COZ(CH2)nxe2x80x94 wherein Z is an oxy or xe2x80x94NHxe2x80x94 group and n is an integer of 1 to 6 (preferably n is 1 to 3).
Most preferably, X is phenylene or xe2x80x94CONH(CH2)3xe2x80x94.
Preferably, Xxe2x80x2 is an oxygen atom.
In structure I:
a) If Y is an imino group, Y can be represented by xe2x80x94Nxe2x95x90CR1R2 wherein R1 and R2 are independently hydrogen, a substituted or unsubstituted alkyl group having 1 to 10 carbon atoms (such as methyl, ethyl, isopropyl, n-hexyl and n-butyl), a substituted or unsubstituted acyl group having 2 to 10 carbon atoms (such as acetyl), or a substituted or unsubstituted carbocyclic or heterocyclic aromatic group (such as phenyl, naphthyl and anthryl). Alternatively, R1 and R2 taken together can provide the atoms necessary to form a substituted or unsubstituted alicyclic ring having from 5 to 15 carbon, oxygen, nitrogen or sulfur atoms in the ring, such as cyclohexyl, cyclohexenyl, tetralonyl and fluorenyl. Such ring structures are usually nonaromatic in character. Preferably, R1 and R2 are taken together to provide the atoms necessary to form an alicyclic ring having from 5 to 15 carbon atoms in the ring, and more preferably they form an unsubstituted alicyclic ring having from 6 to 14 carbon atoms in the ring.
Preferably, Y is an imino group as defined above, and is derived from xcex1-tetralone, fluorenone or cyclohexenone.
b) If Y is an alkyl group, Y can be represented by xe2x80x94CHR3CHR4R5 wherein R5 is an electron withdrawing group, and R3 and R4 are independently hydrogen or a substituted or unsubstituted alkyl group having 1 to 10 carbon atoms (as illustrated above).
An electron withdrawing group is generally known to have a positive Hammett sigma value, and preferably a Hammett sigma value greater than 0.06. Hammett sigma values can be calculated using standard procedures described for example, in Steric Effects in Organic Chemistry, John Wiley and Sons, Inc., 1956, pp. 570-574, and in Progress in Physical Organic Chemistry, Vol.2, Interscience Publishers, 1964, pp. 333-339. Representative useful electron withdrawing groups include, but are not limited to, cyano, sulfo, carboxy, nitro, halo (for example, fluoro and chloro), trihaloalkyl (such as trichloromethyl), trialkylammonium, carbamoyl, sulfamoyl, sulfinyl, pyridinyl, a substituted or unsubstituted aryl group having 6 to 10 carbon atoms in the ring (particularly aryl groups substituted with one or more electron withdrawing groups), sulfinyl and pyridinyl. Preferably, the electron withdrawing group used is sulfo, carboxy, nitro, or a substituted or unsubstituted aryl group, and most preferably, it is sulfo or phenyl.
c) If Y is an amide group, Y can be represented by xe2x80x94NHCOR wherein R represnets an aliphatic group having 1-12 carbon atoms (such as methyl, ethyl, isopropyl, n-hexyl and n-butyl), an aryl having 6-12 carbon atoms (such as phenyl, naphthyl, and anthryl), or a heteroaryl group having 4-12 carbon atoms (pyridyl, thiophyl, and pyrrolyl).
As the sulfonate group is generally pendant to the backbone, preferably it is part of an ethylenically unsaturated polymerizable monomer that can be polymerized using conventional techniques to form vinyl homopolymers of the sulfonate-containing recurring units, or vinyl copolymers when copolymerized with one or more additional ethylenically unsaturated polymerizable monomers. In all instances, the sulfonate-containing recurring units comprise at least 25 mol % of all recurring units in the polymer, and preferably, they comprise from about 40 to 100 mol % of all recurring units. The polymers can include more than one type of repeating unit containing a sulfonate group as described herein.
Polymers having the above-described iminosulfonate group are thought to switch to hydrophilic sulfonic acid under UV irradiation through the following mechanism, as described in xe2x80x9cMicroelectronics Technology: Polymers for Advanced Imaging and Packagingxe2x80x9d (Chapter 21, pp. 318-332; Reichmanis, et al, Eds. ACS 614,1995): 
The liberated free radical II undergoes further decomposition to generate by-product ketone III or two radicals combine to form azine IV. The polymer in the exposed areas is converted to hydrophilic sulfonic acid and rejects ink (in the presence of water) but the unexposed areas are hydrophobic and more readily accept ink. Hence, the imaging member is a positive-working imaging member. Some of the useful sulfoimino groups include: 
Such monomers can be prepared from a reaction between a sulfonic acid halide V with an oxime VI in the presence of a base (Shirai et al. J Polym. Sci., Part C: Polym. Lett. 1986, Vol. 24, pp. 119-224) as illustrated below: 
wherein R6 is hydrogen, an aliphatic group having 1 to 6 carbon atoms or halo.
Sulfonic acid halide V may be easily prepared from the corresponding sodium or potassium salt of sulfonic acid (Kamogawa et al Bull. Chem. Soc. Jpn. 1983, Vol. 56, pp. 762-765) and oxime VI from ketone III (13allini et al Chem. Lett. 1997, pp. 475-476).
The polymers containing Y as an alkyl group undergo a pyrolytic elimination as taught in xe2x80x9cAdvanced Organic Chemistryxe2x80x9d (pp. 1006-1010, March, J. John Wiley and Sons, New York, 1992, 4th ed.), as illustrated below for a representative preferred pendant sulfoalkyl group: 
Hydrophilic sulfonic acid and elimination by-product VIII are generated.
Such polymers can be easily prepared from a monomer containing a sulfonate group represented by the general formula 
This above-said monomers may be prepared from a reaction between a sulfonic acid halide V with an alcohol X in the presence of a base (Organic Synthesis Collective Vol. 5, p. 366), illustrated as follows: 
Representative synthetic methods for making ethylenically unsaturated polymerizable monomers and polymers useful in the practice of this invention are illustrated as follows:
xcex1-Tetralone oxime (24 g) was dissolved in 150 ml of dichloromethane in a 500 ml round-bottomed flask, and cooled to 0 C. Triethylamine (23 ml) was added first and then p-styrenesulfonyl chloride (30.4 g) was added slowly to the solution to keep temperature below 5xc2x0 C. The reaction mixture was stirred at 0xc2x0 C. for 5 hours and then poured into 100 ml of ice-cold 10% HCl solution. The mixture was extracted with dichloromethane three times (100 ml each) and the combined organic layer was washed with water and brine and dried over anhydrous magnesium sulfate. The solvent was removed and the brown solid residue was recrystallized from hexane to obtain off-white crystalline xcex1-tetralone oxime p-styrene sulfonate.
Methyl methacrylate (1.4 ml), xcex1-tetralone oxime p-styrene sulfonate (4.5 g) and azobisisobutylronitrile (hereafter referred to as AIBN, 60 mg) were dissolved in 8 ml of benzene in a 25 ml round-bottomed flask capped with a rubber septum. The solution was purged with dry nitrogen for 15 minutes and then heated at 60xc2x0 C. for 14 hours. The product almost solidified and was diluted with 20 ml of dimethylformamide (hereafter referred to as DMF) and 10 ml of tetrahydrofuran (hereafter referred to as THF). The polymer was precipitated into 400 ml of methanol twice. The resulting white powdery polymer was collected by filtration and dried under vacuum at 40xc2x0 C. overnight.
xcex1-Tetralone oxime p-styrene sulfonate (3.7) g) and AIBN (39 mg) were dissolved in 8 ml of toluene in a 25 ml round-bottomed flask capped with a rubber septum. The solution was purged with dry nitrogen for 10 minutes and then heated at 60xc2x0 C. for 20 hours. The solidified product was diluted with 30 ml of DMF and precipitated into 300 ml of ether. The resulting white powdery polymer was collected and dried under vacuum at 40xc2x0 C. overnight.
Methyl methacrylate (0.52 ml), xcex1-tetralone oxime p-styrene sulfonate (2.0 g), (methacryloyloxy)ethyl acetoacetate (0.19 ml), and AIBN (39 mg) were dissolved in 4.5 ml of benzene in a 25 ml round-bottomed flask capped with a rubber septum. The solution was purged with dry nitrogen for 15 minutes and then heated at 60xc2x0 C. for 15 hours. The product was diluted with 20 ml of DMF and purified by precipitated into 200 ml of methanol. The white powdery polymer was filtered and dried under vacuum at 40xc2x0 C. overnight.
A procedure like that described in Synthesis Example 1 was followed. xcex1-Tetralone oxime (6.0 g) was reacted with 3-methacryloylpropane sulfonyl chloride (7.8 g) in 17 ml of pyridine to give a white crystalline product which was recrystallized from mixed solvent of hexane and diethyl ether.
xcex1-Tetralone oxime 3-methacryloylpropane sulfonate (1.5 g) and AIBN (14 mg) were dissolved in 3 ml of toluene in a 25 ml round-bottomed flask capped with a rubber septum. The solution was purged with dry nitrogen for 10 minutes and then heated at 60xc2x0 C. for 16 hours. The solidified product was diluted with 15 ml of DMF and precipitated into 150 ml of methanol. The resulting white powdery polymer was collected and dried under vacuum at 40xc2x0 C. overnight.
A procedure like that described in Synthesis Example 1 was followed. 2-(Methylsulfonyl)ethanol (6 g) was reacted with p-styrenesulfonyl chloride (11.8 g) in 50 ml of pyridine to give a white crystalline product that was recrystallized from diethyl ether.
2-(Methylsulfonyl)ethanol (4g) and AIBN (14 mg) were dissolved in 15 ml of DMF in a 25 ml round-bottomed flask capped with a rubber septum. The solution was purged with dry nitrogen for 10 minutes and then heated at 60xc2x0 C. for 20 hours. The viscous product was diluted with 15 ml of DMF and precipitated into 400 ml of methanol. The resulting white powdery polymer was collected and dried under vacuum at 40xc2x0 C. overnight.
2-Phenylethanol (3.1 g) was reacted with p-styrene sulfonyl chloride (5.3 g) and triethylamine (2.7 g) in 20 ml of dichloromethane for 3 hours to give the desired product as a light yellow viscous oil that was purified by passing through basic aluminum oxide.
2-Cyanoethylanol (1.8 g) was reacted with p-styrene sulfonyl chloride (5.1 g) and triethylamine (2.5 g) in 20 ml of dichloromethane for 3 hours to give the desired product as a light yellow viscous oil.
Monomer 4 (1.5 g) and AIBN (16 mg) were dissolved in 4 ml of benzene. The solution was then purged with dry nitrogen for 10 minutes and heated at 60xc2x0 C. for 14 hours. The resulting viscous product was diluted to 10 ml with DMF and precipitated into 100 ml of isopropanol. The resulting desired polymer was collected and dried under vacuum at 40xc2x0 C. overnight.
Monomer 4 (2.0 g), methoxymethyl methacrylamide (0.3 g) and AIBN (33 mg) were dissolved in 5 ml of DMF. The solution was purged with dry nitrogen for 10 minutes and heated at 60xc2x0 C. for 14 hours. The resulting viscous product was diluted to 10 ml with DMF and precipitated into 100 ml of diethyl ether. The resulting desired polymer was collected and dried under vacuum at 40xc2x0 C. overnight.
Monomer 4 (2.0 g), 1-vinyl-2-pyrrolidinone (0.3 g) and AIBN (33 mg) were dissolved in 5 ml of DMF. The solution was purged with dry nitrogen for 10 minutes and heated at 60xc2x0 C. for 14 hours. The viscous product was diluted to 10 ml with DMF and precipitated into 100 ml of diethyl ether. The resulting desired polymer was collected and dried under vacuum at 40xc2x0 C. overnight.
Useful additional ethylenically unsaturated polymerizable monomers include, but are not limited to, acrylates (including methacrylates) such as ethyl acrylate, n-butyl acrylate, methyl methacrylate and t-butyl methacrylate, acrylamides (including methacrylamides), an acrylonitrile (including methacrylonitrile), vinyl ethers, styrenes, vinyl acetate, dienes (such as ethylene, propylene, 1,3-butadiene and isobutylene), vinyl pyridine and vinylpyrrolidone.
A mixture of heat-sensitive polymers described herein can be used in the imageable layer of the imaging members, but preferably only a single polymer is used. The polymers can be crosslinked or uncrosslinked when used in the imageable layer. If crosslinked, the crosslinkable moiety is preferably provided from one or more of the additional ethylenically unsaturated polymerizable monomers. The crosslinking cannot interfere with the transformation of the sulfonyl-containing group into a sulfonic acid group during imaging.
The surface imageable layer of the imaging member can include one or more of such homopolymers or copolymers, with or without minor (less than 20 weight % based on total layer dry weight) amounts of additional binder or polymeric materials that will not adversely affect imaging properties of the imageable layer. However, the surface imageable layer includes no additional materials that are needed for imaging, especially those materials conventionally required for wet processing with alkaline developer solutions.
The amount of heat-sensitive polymer(s) used in the imageable layer is generally at least 0.8 g/m2, and preferably from about 1 to about 2 g/m2 (dry weight). This generally provides an average dry thickness of from about 0.1 to about 10 xcexcm. Greater amounts can be used if desired.
The imageable layer can also include one or more conventional surfactants for coatability or other properties, or dyes or colorants to allow visualization of the written image, or any other addenda commonly used in the lithographic art, as long as the concentrations are low enough so that there is no significant interference with layer imaging properties.
The heat-sensitive composition in the imageable layer also includes one or more photothermal conversion materials to absorb appropriate thermal energy from an appropriate source, such as a laser or thermal head, which radiation is converted into heat. Thus, such materials convert photons into heat phonons. Preferably, the radiation absorbed is in the infrared and near-infrared regions of the electromagnetic spectrum. Such materials can be dyes, pigments, evaporated pigments, semiconductor materials, alloys, metals, metal oxides, metal sulfides or combinations thereof, or a dichroic stack of materials that absorb radiation by virtue of their refractive index and thickness. Borides, carbides, nitrides, carbonitrides, bronze-structured oxides and oxides structurally related to the bronze family but lacking the WO2.9 component, are also useful. One particularly useful pigment is carbon of some form (for example, carbon black). The size of the pigment particles should not be more than the thickness of the layer. Preferably, the size of the particles will be half the thickness of the layer or less. Useful absorbing dyes for near infrared diode laser beams are described, for example, in U.S. Pat. No. 4,973,572 (DeBoer), incorporated herein by reference. Particular dyes of interest are xe2x80x9cbroad bandxe2x80x9d dyes, that is those that absorb over a wide band of the spectrum. Mixtures of pigments, dyes, or both, can also be used. Particularly useful infrared radiation absorbing dyes include bis(dichlorobenzene-1,2-dithiol)nickel(2:1)tetrabutyl ammonium chloride, tetrachlorophthalocyanine aluminum chloride, as well as those illustrated as follows: 
IR Dye 2 Same as Dye 1 but with C3F7CO2xe2x88x92 as the anion. 
The photothermal conversion material(s) are generally present in an amount sufficient to provide an optical density of at least 0.3, and preferably at least 1.0. The particular amount needed for this purpose would be readily apparent to one skilled in the art, depending upon the specific material used.
The heat-sensitive composition is coated onto the support using any suitable equipment and procedure, such as spin coating, knife coating, gravure coating, dip coating or extrusion hopper coating.
The imaging members of this invention can be of any useful form including, but not limited to, printing plates, printing cylinders, printing sleeves and printing tapes (including flexible printing webs). Preferably, the imaging members are printing plates.
Printing plates can be of any useful size and shape (for example, square or rectangular) having the requisite heat-sensitive imageable layer disposed on a suitable support. Printing cylinders and sleeves are rotary printing members having the support and heat-sensitive layer in a cylindrical form. Hollow or solid metal cores can be used as substrates for printing sleeves.
During use, the imaging member of this invention can be exposed to any suitable source of thermal energy, such as a focused laser beam or thermal head, in the imaged areas, typically from digital information supplied to the imaging device. No heating, wet processing (such as with an alkaline developer), or mechanical or solvent cleaning is needed before the printing operation (although wiping or cleaning can be used if desired). A vacuum dust collector may be useful during the laser exposure step to keep the focusing lens clean. Such a collector is described in U.S. Pat. No. 5,574,493 (Sanger et al). A laser used to expose the imaging member of this invention is preferably a diode laser, because of the reliability and low maintenance of diode laser systems, but other lasers such as gas or solid state lasers may also be used. The combination of power, intensity and exposure time for laser imaging would be readily apparent to one skilled in the art. Good printing steps are defined as those having a uniform optical density greater than 1.0. Specifications for lasers that emit in the near-IR region, and suitable imaging configurations and devices are described in U.S. Pat. No. 5,339,737 (Lewis et al), incorporated herein by reference. A laser typically emits in the region of maximum responsiveness in the imaging member, that is where the xcexmax closely approximates the wavelength were the imaging member absorbs most strongly.
The imaging apparatus can operate on its own, functioning solely as a platemaker, or it can be incorporated directly into a lithographic printing press. In the latter case, printing may commence immediately after imaging, thereby reducing press set-up time considerably. The imaging apparatus can be configured as a flatbed recorder or as a drum recorder, with the imaging member mounted to the interior or exterior cylindrical surface of the drum.
In the drum configuration, the requisite relative motion between the imaging device (such as a laser beam) and the imaging member can be achieved by rotating the drum (and the imageable member mounted thereon) about its axis, and moving the imaging device parallel to the rotation axis, thereby scanning the imageable member circumferentially so the image xe2x80x9cgrowsxe2x80x9d in the axial direction. Alternatively, the imaging device can be moved parallel to the drum axis and, after each pass across the imageable member, increment angularly so that the image xe2x80x9cgrowsxe2x80x9d circumferentially. In both cases, after a complete scan, an image corresponding (positively or negatively) to the original document or picture can be applied to the surface of the imaging member.
In the flatbed configuration, a laser beam is drawn across either axis of the imageable member, and is indexed along the other axis after each pass. Obviously, the requisite relative motion can be produced by moving the imaging member rather than the laser beam.
Regardless of the manner in which the laser beam is scanned, it is generally preferable (for on-press uses) to employ a plurality of lasers and to guide their outputs to a single writing array. This array is then indexed, after completion of each pass across or along the imaging member, a distance determined by the number of beams emanating from the array, and by the desired resolution (that is, the number of image points per unit length). Off-press applications, which can be designed to accommodate very rapid plate movement and thereby utilize high laser pulse rates, can frequently utilize a single laser as an imaging source.
While laser imaging is preferred in the practice of this invention, any other imaging means can be used that provides thermal energy that can be directed in an imagewise fashion. For example, imaging can be accomplished using a thermoresistive head (or thermal printing head) in what is known as thermal printing, as described for example, in U.S. Pat. No. 5,488,025 (Martin et al). Such thermal printing heads are commercially available (for example as Fujitsu Thermal Head FTP-040 MCS001 and TDK Thermal Head F415 HH7-1089).
Without any wet processing steps (such as processing with an alkaline developer) after imaging, the imaging member is then used for printing by applying a lithographic ink to the image on its surface, in the presence of a fountain solution, and by transferring the ink to a suitable receiving material (such as cloth, paper, metal, glass or plastic) to provide a desired impression of the image thereon. An intermediate xe2x80x9cblanketxe2x80x9d roller can be used in the transfer of the ink from the imaging member to the receiving material. The imaging members can be cleaned between impressions, if desired, using conventional cleaning means. Thus, imaging and printing can be carried out without conventional xe2x80x9cwetxe2x80x9d processing. Hence, the imaging members of this invention are considered xe2x80x9cprocesslessxe2x80x9d imaging members.
The following examples illustrate the practice of the invention, and are not meant to limit it in any way.
In these examples, a thermal IR-laser platesetter was used to image the printing plates, the printer being similar to that described in U.S. Pat. No. 5,168,288 (Baek et al), incorporated herein by reference. The printing plates were exposed using approximately 450 mW per channel, 9 channels per swath, 945 lines/cm, a drum circumference of 53 cm and an image spot (1/e2) at the image plane of about 25 micrometers. The test image included text, positive and negative lines, half tone dot patterns and a half-tone image. Images were printed at speeds up to 1100 revolutions per minute (the exposure levels do not necessarily correspond to the optimum exposure levels for the tested printing plates).